Coextrusion of PVC (polyvinyl chloride) allows various types of PVC to be used in different zones of a workpiece. For example, coextrusion has been used to bond a capstock material in a relatively thin exposed superficial layer, to a substrate material which can be a PVC foam.
In the past, for the coextrusion of generally flat workpieces, such as siding, it has been the practice to feed the two streams of PVC to the outlet of a coextrusion block in a pattern in which, over the length of the passages for the materials in the coextrusion block, the flows were generally of rectangular cross section.
When the two flows emerged at the circular outlet, the capstock material occupied a segment separated from the body of substrate material by a chord of the circular outlet.
Coextrusion of PVC for the production of siding in this fashion has been found to involve problems with respect to burn up. Heretofore, because of the way in which the two flows were generated in the extrusion block and met at the outlet, flow angles were considerable and back pressure at high output rates generated high temperatures resulting in damage to the PVC materials and the need to carefully control the compositions of such materials and counteract the tendency to burn up by adjustment of the composition. The need to carefully control the composition to prevent burn up, of course, greatly reduced the versatility of the apparatus.